The present invention is directed to improvements in driers and methods of drying used to dry various materials, including newly harvested grain, wood pellets, etc. and, in particular, to driers that recover and utilize a comparatively high percentage of the energy used in the drying process.
The drying industry is very large and utilizes significant amounts of both fossil fuels and electricity to dry various materials. While the grain industry is not the only industry that requires significant drying, it is indicative of the problems that exist. Just the U.S. corn crop amounts to almost nine billion bushels annually.
Moisture must be removed in order to allow the grain to be stored without significant loss due to mold, mildew and rot caused by excess retained moisture.
In theory, each pound of water removed from the grain has a latent heat of vaporization of about 1160 British thermal units (Btu's) per pound. In a highly effective dryer system, the dryer could import exactly this theoretical amount of energy per pound of water to be removed from the grain. In reality, the grain also takes on sensible heat and raises in temperature during the process, the flow of heating gas is not uniform, the grain is often heated more on one side of the dryer than the other, etc., such that the efficiency of all types of conventional driers is comparatively low. Cross flow grain driers normally require approximately 2800 Btu per pound of water removed versus the theoretical amount of 1160 Btu per pound.
Because the corn industry in the U.S. consumes approximately 900 million gallons of propane and over 3200 million kilowatt-hours of electricity per year just to dry the corn and because this produces nearly two million tons of carbon dioxide exhaust gases per year, it is seen that any improvement in drying efficiency can amount to significant savings in fuel, energy and emissions. Corn is only one type of grain that must be dried. Further, there are many other solids, semi-solids and initially liquid compositions that are dried each year by vaporizing a liquid component or completely evaporating most or all of an incoming stream, at considerable costs in terms of fuel, energy and undesired emissions due to combustion of the fuels.
It is further noted that for some materials the manner of drying is important to prevent excessive shock to the product being dried and/or to reduce inconsistency in the dried material. For example, grain kernels can be cracked by cooling or heating too quickly, which can lead to degradation of the grain. While conventional driers may provide a chosen average moisture content, the content may not be consistent. Consequently, problems are encountered in many types of conventional cross flow grain driers where, the grain is heated and dried by air passing perpendicularly to the flow of the grain. In such driers, the grain on one side of the dryer that first encounters the heated air is overly dried and may be dried too quickly or cooled too quickly so as to cause cracking and the grain on the opposite or on air discharge side tends to be too wet. Therefore, it is also desirable to provide a dryer that provides consistent, uniform and non stressful heating to drive off moisture and thereafter uniform and non stressful cooling.
In some circumstances, it is also desirable to provide a closed recycle system for gas used in the drying process to reduce dust or other undesirable emissions.